NAND flash memory

NAND flash memory is a type of nonvolatile storage technology that does not require power to retain data. An important goal of NAND flash development has been to reduce the cost per bit and to increase maximum chip capacity so that flash memory can compete with magnetic storage devices, such as hard disks. NAND flash has found a market in devices to which large files are frequently uploaded and replaced. MP3 players, digital cameras and USB flash drives use NAND technology.

NAND flash saves data as blocks and relies on electric circuits to store data. When power is detached from NAND flash memory, a metal-oxide semiconductor will provide an extra charge to the memory cell, keeping the data. The metal-oxide semiconductor typically used is a floating-gate transistor (FGT). The FGTs are structured similar to NAND logic gates.

NAND memory cells are made with two types of gates, control and floating gates. Both gates will help control the flow of data. To program one cell, a voltage charge is sent to the control gate.

Vendors in NAND flash memory include Samsung, Toshiba, Intel, Western Digital and Micron Technology.

NAND flash memory operation

NAND has a finite number of write cycles. NAND failure is usually gradual, as individual cells fail and overall performance degrades, a concept known as wear-out. To help compensate, some vendors overprovision their systems by including more memory than is actually claimed.

When a NAND card wears out, the user simply buys a new one, and the device continues to function. By passing the expense of additional storage on to the consumer, manufacturers have been able to lower the price of consumer electronic devices significantly. New developments in NAND flash memory technology are making the chips smaller, increasing the maximum read-write cycles and lowering voltage demands.

NAND flash can only support a small number of write cycles per block. It delivers speedy read access, but it is not as fast as static read-only memory (ROM) or random access memory (RAM). The technology is shock resistant and can withstand high and low temperatures, as well as submersion in water, so it performs better than hard disks in mobile devices.

Types of NAND flash storage

Common types of NAND flash storage includes SLC, MLC, TLC, QLC, and 3D NAND. What separates each type is the number of bits each cell uses. The more bits stored in each cell, the less expensive the NAND flash storage would cost.

  • SLC, or single-level cells, store one bit in each cell. SLC has the highest endurance but is also the most expensive type of NAND flash storage.
  • MLC, or multi-level cells, stores two bits in each cell. Because erasures and writes occur two times more, MLC has less endurance compared to SLC. However it is less expensive. Many PCs will use MLC.
  • TLC, or triple-level cells, store three bits in each cell. Many consumer-level products will use this since it is less expensive, however lower performing.
  • QLC, or quad-level cells, store four bits in each cell. QLCs have even less endurance and are generally less expensive.
  • 3D NAND. 2D or Planar NAND has only one layer of memory cells, whereas 3D NAND stacks cells on top of one another. Samsung refers to 3D NAND as Vertical NAND or V-NAND.

NAND flash shortage

A NAND flash shortage began in 2016. The shortage is partially the result of demand, but it is also because vendors are transitioning from manufacturing 2D or planar NAND to much denser 3D NAND technology. Making 3D NAND chips is a more complicated process.

NAND flash vs. NOR flash

The two main kinds of flash are NAND and NOR flash memory, which get their names from their respective logic gates.

NAND flash memory is written and read in blocks that are smaller than the device, whereas NOR flash memory reads and writes bytes independently. Use cases for both NOR and NAND flash memory include laptop and desktop computers, digital cameras and audio players, smartphones, video games and scientific, industrial and medical electronics.

NAND flash offers faster erase and write times than NOR flash, while NAND technology delivers better density at a lower cost per bit. NAND also offers up to ten times the endurance or NOR.

NAND is not a suitable replacement for ROM because it does not deliver byte-level random access, which data stored on ROM usually requires. NOR memory makes a good replacement for RAM and ROM drives. NAND is more closely related to secondary storage devices, such as hard disks. This makes it good for mass storage use cases.

Memory types comparison

This was last updated in July 2019

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